The range properties and range limitation of an impedance protection device, according to the prior art, are related to the so-called model impedance which is directly or indirectly included in such protection devices. In the event of a load, the impedance of which falls within an impedance range determined with the aid of the model impedance in an R-X plane, this is assumed to imply a fault on the network. The range properties are then coupled to the resistance and inductance values of the model impedance, which values can be adjusted individually. This then means that the impedance range which implies a fault on the network may have different shapes. A protection device based on this principle and with analog components is described, inter alia, in U.S. Pat. No. 3,629,658 (Helge, Johansson). This protection device includes, among other things, analog filters, which have limitations in the area of selectivity and in their speed of action. The provision of the desired protective function characteristic, among other things range limits, may be a difficult problem due, for example, to the frequency dependence of the components included.
U.S. patent application Ser. No. 696,634, filed on Jan. 30, 1985 in the name of Brandt, also describes a protection device based on the same principle but designed with digital components.